The present invention relates to synchronizing means for use in a hydraulic circuit of the type including a plurality of hydraulic motors operable by separate control valves with fluid under pressure for the various valves and motors being supplied by a single pump. More particularly, the invention relates to such a hydraulic circuit wherein the synchronizing means includes a single element for operating the pump, the single element being responsive to operation of any one of the control valves.
Hydraulic circuits of the type contemplated by the present invention are commonly employed, for example, in material handling vehicles such as fork lift trucks where the vehicle includes a number of hydraulic motors or jacks which must be separately operable to regulate material handling implements associated with the vehicle. One particular example of such a combination of motors and separate valves may be seen in the conventional mast unit of a fork lift truck. A number of hydraulic jacks may be employed in conjunction with such mast units to accomplish a variety of functions including raising and lowering of a carriage assembly upon the mast unit, tilting or "racking back" the carriage and mast unit to secure a load upon the carriage, operating clamping mechanisms commonly associated with the carriage or to transversely position the carriage in a side shifting mode for alignment with a load.
Numerous other examples are available of vehicles or machines including a number of hydraulic motors which may be operated by a single hydraulic pump. It is sufficient to realize that the synchronizing unit of the present invention may be adapted to any such vehicle or machine without limitation.
Generally, it is well known to have separate control valves for separate hydraulic motors and to supply fluid under pressure to all of the control valves from a single hydraulic pump. In this regard, reference is particularly made, for example, to U.S. Pat. No. 3,593,523 and U.S. Pat. No. 3,589,130. Each of these patents relates to such a hydraulic circuit wherein actuating fluid under pressure for a plurality of hydraulic motors is provided by a single pump. However, these references are also illustrative of the relatively complex problem of assuring accurate synchronization between operation of the pump and movement of any one of the control valves for the separate motors to accomplish its operation. The plurality of control valves contemplated for use within such a hydraulic circuit are often of a type commonly referred to as "closed center" valves. These valves have "dead bands" in their neutral positions so that the control valve must be moved a discrete distance out of its neutral condition before communication is established with its motor. Such valves present a particular problem in synchronization since it is desirable to initiate operation of the pump at precisely the time when the control valve passes the dead band and commences to open fluid communication to the motor.
An additional problem is encountered in such hydraulic circuits since there is often only a limited amount of space available for the control valves and associated components. For example, in fork lift trucks, it is desirable to arrange these components in a relatively compact console at the operator station. Actuating elements or control levers for the various valves may then be positioned for ready access by an operator. Accordingly, it is necessary that the controls for such a hydraulic circuit not only permit close synchronization between operation of the valves and the single pump but also that the entire control unit be of relatively simple and reliable design in addition to forming a compact unit.